1. Field of the Invention
The present invention relates to a process for preparing 5-ethylidene-2-norbornene (hereinafter referred to as "ENB"). Particularly, it relates to a process for preparing ENB by isomerizing 5-vinyl-2-norbornene (hereinafter referred to as "VNB") in the presence of a specific solid base catalyst.
2. Description of the Prior Art
ENB is the most promising compound as a third monomer of a terpolymer of ethylene, propylene and dienemonomer (EPDM rubber) and prepared by isomerizing VNB in the presence of a catalyst. VNB is produced by reacting 1,3-butadiene and cyclopentadiene.
As the isomerization catalysts, there are known liquid bases such as mixtures of an alkali metal hydroxide and an aprotic organic solvent, of an alkali metal amide and an amine, and of an organic alkali metal compound and an aliphatic amine. Such liquid bases, however, do not have enough catalytic activity so that a large amount of the expensive catalyst should be used. Further, since separation and recovering of the catalyst component from the reaction mixture are very difficult, the process requires complicated separation and recovering steps and consumes a large amount of energy.
There are also known solid isomerizatiion catalysts, for example, an alkali metal carried on an anhydrous carrier with a large surface area (e.g., activated carbon, silica gel, alumina and the like) (cf. J. Am. Chem. Soc., 82, 387 (1960)). The solid catalyst, however, has unsartisfactory handleability and less safety since it is ignited and loses activity on contact with air. This is because the alkali metal is only finely dispersed on the carrier. Further, the solid catalyst has insufficient isomerization performance.
The inventors have proposed a solid base catalyst for isomerizing olefins such as VNB, which do not suffer from the drawbacks of the conventional isomerization catalysts. The proposed catalyst is prepared from alumina, an alkali metal hydroxide and an alkali metal, or from water-containing alumina and an alkali metal. The solid base catalyst has higher stability to air and excellent isomerization activity of olefins such as VNB than the alkali metal dispersion catalyst (cf. Japanese patent publication Nos. 3274/1975 and 21378/1982).